1. Field of the Invention
The present invention relates to optical switches to be used in communication systems based on the propagation of light within waveguides, and especially relates to an optically-controlled grating optical switch in which an ON-OFF of signal light is controlled by a control light.
2. Description of the Related Art
Heretofore, optically-controlled optical switches have been regarded as important devices for realizing all-optical signal processing systems and ultra-high speed optical switching systems. Because of its marked characteristics of a large optical non-linearity of a multiple quantum well (MQW), the applicability of a semiconductor laser as a light source and others, an optical switch using a MQW structure of III-V group compound semiconductor has been investigated and developed as one of the most useful optical switches, and hence some types thereof have been reported. For example a plate Fabry-Perot etalon is disclosed by A. Miqus et al., Appl. Phys. Lett. 46, 70, 1985, and a waveguide grating coupler is disclosed by R. Jin et al., Appl. Phys. Lett. 53, 1791, 1988. In particular, an optically-controlled grating switch (OG-SW) has attracted special interest for its excellent switching properties (Japanese Patent Application Laying-open No. 182833/1989).
One example of the conventional optically-controlled grating switch will be explained by the following description referring to the accompanied drawings FIGS. 1-5.
FIG. 1 is a schematic perspective view for demonstrating a typical structure of the conventional optical switch, while FIG. 2 shows its plan view. As shown in these figures, the OG-SW includes an input/output optical waveguide(s) for signal light 1 (1A and 1B, respectively), a grating switch region 2 which is responsible for switching a signal light transmission and a signal light reflection, a control light optical waveguide 3, an InP cladding 4, and an InP substrate 5.
FIGS. 3, 4 and 5 are cross sectional views along lines A-A', B-B' and C-C' in FIGS. 1 and 2, respectively. The input/output optical waveguides 1A and 1B are constructed of cores which are made of In.sub.0.72 Ga.sub.0.28 AS.sub.0.59 P.sub.0.41. The grating 2 is constructed of an optical nonlinear medium having a multiple quantum well structure and its inner structure is composed of quantum well layers 6 and barrier layers 7. The quantum well layer 6 consists of a 50 .ANG. thick layer of In.sub.0.47 Ga.sub.0.53 As, while the barrier layer 7 consists of a 75 .ANG. thick InP layer.
The band gap wavelength .lambda..sub.g of the multiple quantum well structure takes a value of 1.50 .mu.m. This bandgap wavelength is defined as an absorption band wavelength .lambda..sub.g which is fixed by the bandgap of the semiconductor. In the case that an incident light wavelength .lambda. is shorter than the absorption band wavelength .lambda..sub.g (i.e., .lambda.&lt;.lambda..sub.g), the incident light is absorbed. In the case that the incident light .lambda. is longer than the absorption band wavelength .lambda..sub.g (i.e., .lambda.&gt;.lambda..sub.g), on the other hand, the incident light is transmitted. This optical switch is constructed so that an optical axis of the grating 2 is coincident with that of the core 1. The optical axis is defined as a direction of the propagation along a Z axis which is perpendicular to an X axis and a Y axis (these axes are fixed in the three-dimensional space as shown in FIG. 1). The control light optical waveguide 3 is formed as a core which is made of In.sub.0.72 Ga.sub.0.28 As.sub.0.59 P.sub.0.41 and is embedded in the cladding layer 4 as shown in FIG. 4. The construction of the control light optical waveguide 3 is similar to that shown in FIG. 3. In the conventional optical switch, as shown in FIG. 2, the input/output optical waveguide 1 and the control light optical waveguide 3 are positioned such that they perpendicularly intersect each other and thus the signal light and the control light are injected into the grating 2 from the directions which perpendicularly intersect each other.
Next, a switching action of the conventional optical switch constructed as described above will be explained. In this case, the switching action using a signal light wavelength of 1.55 .mu.m which is also used in the examples of the present invention, will be described for the purpose of making an easy comparison with the present invention.
The pitch or the like of the grating 2 is determined by a wavelength band of the signal light which is defined as the Bragg wavelength .lambda..sub.B. In the case of using .lambda..sub.B =1.55 .mu.m, for example, the grating should be constructed as a first-order diffraction grating to perfectly reflect the light at a wavelength of 1.55 .mu.m, but to transmit the light at a wavelength in the neighborhood of 1.55 .mu.m. There is a certain relationship between the Bragg wavelength .lambda..sub.B and a pitch .LAMBDA. of the grating. As in a correspondence with the Bragg wavelength .lambda..sub.B =1.55 .mu.m, a pitch and a thickness of the grating in the conventional example are fixed at values of 0.24 .mu.m and 0.34 .mu.m, respectively. Reflection coupling coefficient .kappa. is defined by a degree of periodical changes of a refractive index and a percentage of confinement of the light in the region where the refractive index changes periodically, and also it indicates a degree of coupling between a forward wave (incident light) and a backward wave (reflected light). Therefore, the refractive indices of the optical nonlinear medium 6 and the cladding 5 are 3.4 and 3.2, respectively, so that the .kappa. value takes about 300 cm.sup.-1 or more when the grating 2 is formed to have 0.15 .mu.m depth, and also the light can be reflected almost perfectly (about 99%) when the grating 2 is formed to have 100 .mu.m length (L.sub.g).
FIG. 6 shows a calculated wavelength-reflectance characteristics of the grating 2. The grating 2 is constructed to have a reflectance profile as indicated by a curve A having a peak value at a wavelength corresponding to the signal light wavelength, .lambda..sub.S =1.55 .mu.m. Therefore, the Bragg wavelength .lambda..sub.B of the grating 2 corresponds to the signal light wavelength .lambda..sub.S. When the control light at a certain wavelength .lambda..sub.C is incident upon the grating 2, the Bragg wavelength is shifted .DELTA..lambda. from the signal light wavelength .lambda..sub.S as indicated by a curve B such that the reflectance takes a value of zero at the signal light wavelength .lambda..sub.S. As the result, the signal light passes through the grating.
The shift of the Bragg wavelength is dependent on the excitation of carriers in the optical nonlinear medium 6 by injecting the control light into the grating 2. In this case, a very high speed switching can be attained because transition from the initial state of the switch is determined by an interband transition time of the carriers in the multiple quantum well structure.
As described above, the conventional optically-controlled grating optical switch is constructed by the semiconductor medium having a quantum well structure in which the control light is injected from a direction perpendicular to the input/output optical waveguide. Therefore, the advantages of the conventional switch are as follows:
(i) as the conventional switch is constructed, as in the type of a waveguide structure, it is possible to integrate and miniaturize; PA1 (ii) as the conventional switch is constructed, as in the type of a waveguide structure, the switching operation can be performed by low switching power; PA1 (iii) as the signal light passes along a direction perpendicular to the control light, there is no unexpected results caused by the interaction between them and thus there is no need to use an optical isolator and the like; PA1 (iv) signal light and control light at a certain wavelength can be optionally selected, so that the switch can be easily constructed and performs with a high efficiency; and PA1 (v) there is no need to modify the construction of the optical switch to provide a gate-type optical switch, a bistable memory, a device with a function of waveform-reconstruction or with other highly functions and the like. PA1 a switching region for the ON-OFF switch control by which the signal light transmission and the signal light reflection are switched; PA1 an input/output optical waveguide region for guiding the signal light to the switching region and for outputting the signal light from the switch region; PA1 a coupler region for coupling the control light colinearly to the signal light and for guiding the control light to the switch region together with the signal light; PA1 a separator region for separating the signal light and the control light. PA1 another switching region for switching the signal light; PA1 another input/output optical waveguide region connected to the separator region and the another switch region; PA1 another coupler region for coupling another control light colinearly to the signal light and for guiding the another control light to the another switch region together with the signal light and another separator region for separating the signal light and the another control light. PA1 a semiconductor substrate; PA1 a first optical waveguide for guiding the signal light and formed on the substrate; PA1 a grating provided in the first optical waveguide and made of a monoresonant medium with a multiple quantum well structure for the ON-OFF switch control by which the signal light transmission and the signal light reflection are switched; PA1 a second optical waveguide for guiding control light and formed on the substrate, a part of the second optical waveguide being close to the first optical waveguide to form a directional coupler for coupling the control light colinearly to the signal light and for guiding the control light to the grating together with the signal light; and PA1 a grating coupler for separating the control light and the signal light and connected to the first optical waveguide; PA1 wherein the grating transmits the signal light when the control light is coupled while reflects the signal light when the control light is not coupled, so that the grating acts as a switch.
In spite of these advantages, however, the conventional optical switch requires a switching speed of at least 10 nanoseconds for recovering the initial condition because the switching operation is restricted by a life time of the carriers in the multiple quantum well structure. The switching speed of such order is not sufficient to satisfy the requirement of the fast switching action.
To solve the problem concerning about the switching speed which the conventional optical switch does not solve, a modified optically-controlled grating switch have been proposed in Japanese Patent Application Laid-open No. 208920/1992. In this reference, an optical switch having a switching speed of about 1 nanosecond can be provided by constructing the optical switch to have a grating with a reflection coupling coefficient .kappa. of 500 cm.sup.-1 or an impurity concentration of over 5.times.10.sup.17 cm.sup.-3 in the multiple quantum well structure. In spite of such modification, however, the switching speed is still not remarkably decreased and thus fast switching is not satisfied. Further improvements for obtaining the higher switching speed have been required. In addition, the conventional optical switch comprises a switching region having an absorption coefficient of 50-100 cm.sup.-1 with respect to the signal light while an absorption loss of 3-6 dB, because the signal light incidence is obliged to perform at a wavelength in the neighborhood of the a bandgap wavelength of the multiple quantum well structure.
Accordingly, the novel optically-controlled grating switch having properties of an extremely higher switching speed and a lower absorption loss compared with the conventional switch has been demanded.